Non-linearities which occur in raster scan CRT displays include on-axis or velocity non-linearity, and pin cushion non-linearity. The effects of these non-linearities are illustrated in FIG. 1 and are due primarily to the difference in location of the center of beam deflection and the center of curvature of the CRT face. Pin cushion non-linearity results in a bowing of horizontal and vertical raster dimensions and velocity non-linearity results in variation in raster line separation.
In monochrome CRT displays, velocity non-linearity is generally corrected for by electrically altering the shape of the sweep current to slow sweep velocity at raster edges and pin cushion effects are corrected for by suitable positioning of permanent magnets on the CRT neck. Using this technique, linearities of approximately .+-.0.5% are typically obtained where linearity is defined as ##EQU1## L being line length and .DELTA.d being peak deviation from a straight line drawn between end points.
In color CRT displays having multiple beams, however, permanent magnets cannot be used for error correction since the magnetic field affects each beam differently causing color misconvergence. As a result, pin cushion correction for such color CRT displays is generally done electrically. Side-to-side non-linearities are corrected for by altering horizontal sweep width as function of vertical position. Top to bottom non-linearities are corrected for by altering sweep height as a function of horizontal position. The latter is by far the most difficult task since it essentially involves superimposing a high frequency, horizontal sweep rate, waveform onto a low frequency, frame rate, waveform and then driving a vertical yoke's inductive load. Techniques used in the television industry have generally involved driving the vertical yoke from a resonant circuit tuned to the horizontal sweep rate. The drive waveform is applied through a saturating transformer giving the effect of varying the drive amplitude as a function of sweep height. Such techniques do not offer linearities of much bettern than .+-.2% and are typically difficult to align and maintain stable.
The invention provides a novel solution to top-bottom pin cushion problems and a vertical deflection system which is inherently more stable, provides vertical centering and protection against loss of vertical drive, and produces a linearity for color systems comparable to the linearity of monochrome systems.